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Abstract

The formation of giant and supergiant porphyry deposits is interpreted to be genetically linked to the subduction of major transform structures on the sea floor. The ultramafic crust and lithosphere associated with oceanic transforms and fracture zones undergo high degrees of metasomatic alteration (serpentinization) aided by ongoing rupture and enhanced fluid flow relative to that experienced by adjacent, structurally homogeneous oceanic crust. When the highly serpentinized (fluid-enriched) oceanic crust and lithosphere formed at these fracture zones subducts at convergent plate boundaries, the hydrous, lithospheric-scale structural weaknesses may locally develop into vertical slab tears. The formation of vertical slab tears leads to localized mantle flow and elevated temperatures at the edges of the slab. The ensuing thermal perturbations along the slab edges enhance serpentinite breakdown reactions and aids in the liberation of aqueous fluids at approximately 600°C and 80- to 100-km depth. This pressure and temperature range is close to the wet melting curve, thereby increasing the potential for slab melting and the generation of magmas such as adakites with relatively high log fo2 >FMQ. Large oceanic transforms such as the Mocha-Valdiva fracture zone in the southeastern Pacific are likely to contain the greatest proportion of serpentinite and therefore carry fluid in the form of serpentinite group minerals into the mantle. Ultimately, the volume of fluid liberated during subduction and dehydration of the serpentinized mantle at these fractures far exceeds the volume of fluid produced at adjacent, weakly altered, and “structureless” oceanic crust. We suggest that the combination of extremely large volumes of slab-derived fluids plus the potential for slab melts to develop in these regions represents a primary control on the formation of giant porphyry deposits. These conditions are met along the eastern Pacific Rim, where more than 10 large transforms are currently being subducte. The endowment of the subducting oceanic crust with transforms and equally impressive mineral endowment contrasts markedly with the western Pacific Rim where only three to four equivalent-sized transforms are found. We propose that the variation in mineral endowment between the eastern and western Pacific and the formation of giant and supergiant porphyry deposits is linked to the formation and subduction of large oceanic fractures.

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